1. Field of the Invention
The present invention generally relates to systems for control of an appliance incorporating a flame and more particularly relates to flame management systems.
2. Description of the Prior Art
It is known in the art to employ various appliances for household and industrial applications which utilize a fuel such as natural gas (i.e., methane), propane, or similar gaseous hydrocarbons. Typically, such appliances have the primary heat supplied by a main burner with a substantial pressurized gas input regulated via a main valve. Ordinarily, the main burner consumes so much fuel and generates so much heat that the main burner is ignited only as necessary. At other times (e.g., the appliance is not used, etc.), the main valve is closed extinguishing the main burner flame.
A customary approach to reigniting the main burner whenever needed is through the use of a pilot light. The pilot light is a second, much smaller burner, having a small pressurized gas input regulated via a pilot valve. In most installations, the pilot light is intended to burn perpetually. Thus, turning the main valve on provides fuel to the main burner which is quickly ignited by the pilot light flame. Turning the main valve off, extinguishes the main burner, which can readily be reignited by the presence of the pilot light.
These fuels, being toxic and highly flammable, are particularly dangerous in a gaseous state if released into the ambient. Therefore, it is customary to provide certain safety features for ensuring that the pilot valve and main valve are never open when a flame is not present preventing release of the fuel into the atmosphere. A standard approach uses a thermogenerative electrical device (e.g., thermocouple, thermopile, solar cell, etc.) in close proximity to the properly operating flame. Whenever the corresponding flame is present, the thermocouple generates a current. A solenoid operated portion of the pilot valve and the main valve require the presence of a current from the thermocouple to maintain the corresponding valve in the open position. Therefore, if no flame is present and the thermocouple(s) is cold and not generating current, neither the pilot valve nor the main valve will release any fuel. U.S. Pat. No. 4,988,884, issued to Dunbar et al. shows a thermogenerative device thermally coupled to a flame.
In practice, the pilot light is ignited infrequently such as at installation, loss of fuel supply, etc. Ignition is accomplished by manually overriding the safety feature and holding the pilot valve open while the pilot light is lit using a match or piezo igniter. The manual override is held until the heat from the pilot flame is sufficient to cause the thermocouple to generate enough current to energize the safety solenoid. The pilot valve remains open as long as the thermocouple continues to generate sufficient current to actuate the pilot valve solenoid.
The safety thermocouple(s) can be replaced with a thermopile(s) or other device for generation of additional electrical power. This additional power may be desired for operating various indicators or for powering interfaces to equipment external to the appliance. U.S. Pat. No. 5,931,655, issued to Maher, Jr. and U.S. Pat. No. 4,778,378, issued to Dolnick et al. show generation and usage of such thermally generated power. However, upon loss of flame (e.g., from loss of fuel pressure), the thermocouple(s) ceases generating electrical current and the pilot valve and main valve are closed. The delay from loss of flame until closure of the valves depends upon a number of variables. Of greatest concern is the delay caused by heat energy retained in the appliance, including the thermopile(s). That means that as the size and current generation capacity of the thermopile(s) are increased, the system delays are correspondingly increased.
The present invention overcomes the disadvantages of the prior art by providing a method of and apparatus for providing an earlier indication of a flame out condition. In accordance with the preferred mode of the present invention, a thermopile is utilized to provide sufficient current to power a small microprocessor and a number of other electrical components. One of the functions of the microprocessor is to measure the output voltage of the thermopile and maintain a history of that voltage output. By comparing the instantaneous output voltage to the history, the microprocessor can diagnose a flame out condition from the voltage output signature much earlier than electrical current generation by the thermopile actually ceases.
The preferred embodiment employs a two stage low voltage DC-to-DC converter which converts the thermopile output to power the microprocessor and other electrical components.
Upon being powered up, the microprocessor samples the thermopile output voltage once every second. Every eight seconds an average is calculated. A complete xe2x80x9chistoryxe2x80x9d includes eight averages of eight readings each, covering the last 64 seconds. These readings are arranged in time through storage in a FIFO push down stack. That means that as each new average is calculated, it is entered into the location in the stack for the latest reading. All previous readings are shifted back one place in the stack. The 9th last reading is shifted out of the stack and thus deleted.
The contents of the stack provide a signature of the output voltage versus time curve of the thermopile output. Using the algorithms described below in detail, the flame out condition can be detected much earlier than complete loss of thermopile output.
The thermopile has a certain internal resistance. In the preferred mode of practicing the invention, the main valve shares power from the same thermopile. When the main valve is turned on, the total thermopile output current increases resulting in a lowered thermopile output voltage. The microprocessor is notified of the mode change so that the algorithm can accommodate the mode change without falsely detecting a flame out condition.